1. Field of the Invention
The present invention relates to bidirectional switches for controlling a load connected to the mains. The present invention more specifically applies to "normally-on" switches, that is, switches which are spontaneously in the on-state (i.e., conducting) and the control of which includes opening the switch to block (i.e., disconnect) the load supply.
2. Discussion of the Related Art
FIG. 1 shows a first example of a normally-on bidirectional switch 1. Switch 1 is essentially formed of a triac 11 connected between two power terminals 12, 13. Gate G of triac 11 is connected, via a controllable switch 14, to a first power terminal 12 or first anode A1 of triac 11. A resistor R1 of high value connects gate G of triac 11 to a second power terminal 13 or second anode A2 of the triac, so that the series association of resistor R1 and of switch 14 is connected in parallel on triac 11.
Switch 1 is meant to be connected in series with a load 15 (Q) between two terminals 16, 17 of application of an a.c. supply voltage Vac, for example, the mains voltage.
Control switch 14 can be a manual control switch or receive a control signal CTRL provided by an appropriate control circuit.
The operation of a switch such as shown in FIG. 1 is the following. It is assumed that, at rest, switch 14 is open. When a voltage is applied across switch 1, that is, when an a.c. voltage is applied between terminals 16 and 17, a current flows in gate G of triac 11 through resistor R1 and triggers the triac which remains on as long as it conducts a current, that is, until the zero crossing of the a.c. voltage. This process is repeated for each halfwave of a.c. supply voltage Vac. For example, at the beginning of a positive halfwave, a current flows from terminal 13 through resistor R1, and from gate G to anode A1 of triac 11, until this current is sufficient to trigger the triac in the first quadrant (positive gate and anode currents). Once triac 11 has been triggered, the current flows therethrough until the end of the halfwave, where the triac turns off. At the beginning of a negative halfwave, a current flows, from terminal 12, from anode A1 to gate G of triac 11 and through resistor R1, until this current is sufficient to trigger the triac in the third quadrant (negative anode and gate currents).
Resistor R1 is sized according to the gate current required to trigger the triac and to the maximum acceptable supply voltage (generally on the order of 20 volts) to turn on switch 1 at the beginning of each halfwave. When switch 14 is closed (by an action exterior to switch 1), gate G and anode A1 of the triac are short-circuited and the triac can no longer trigger and remains in the off-state.
A disadvantage of a switch such as shown in FIG. 1 is that, when the triac is maintained in the off-state (switch 14 being closed), resistor R1 dissipates a high power. Indeed, the triggering current of a triac is relatively high, which does not allow use of a high resistance R1 while respecting the imperative of a low voltage triggering which characterizes a normally-on switch. Presently, the triacs which are most sensitive at the triggering require a gate current of several mA. This high triggering current is linked to the structure of a triac. A sufficiently high triggering current to avoid that the residual non-recombined loads in the semiconductor cause a restarting of the triac at halfwave ends must indeed be provided.
For example, among the most sensitive triacs manufactured by SGS-THOMSON Microelectronics, the triacs known under denomination Z0103 and Z0402 require a gate current of 3 mA to be triggered.
With such a minimum gate current value and for a triggering voltage of 20 volts, a resistance R1 on the order of 7 k.OMEGA. has to be provided. This results in a dissipated power on the order of 6 watts when switch 14 is closed and when voltage Vac is the 220 V mains voltage.
The implementation of a normally-on bidirectional switch in which the dissipated power is substantially lower than with the switch of FIG. 1 has already been provided. FIG. 2 shows an example of such a normally-on bidirectional switch 2, connected in series with a load 25 between two terminals 26, 27 of application of an a.c. supply voltage Vac.
As previously, a triac 21 is connected between two power terminals 22, 23 of switch 2 to which are respectively connected a terminal (for example, 27) of application of the supply voltage and a first terminal of load 25. Gate G of triac 21 is connected to an a.c. input of a diode bridge 28, the other a.c. input of which is connected to terminal 23, and thus to an anode (for example, A2) of triac 21. A resistor R2, in series with a switch 24 of control of switch 2, is connected between the (+) and (-) rectified voltage terminals of bridge 28. A thyristor 29 is connected in parallel to the series association of resistor R2 and switch 24, its anode being connected to the positive rectified voltage terminal (+) of bridge 28 and its cathode being connected to the negative terminal (-). The gate of thyristor 29 is connected to the connection node of resistor R2 and switch 24.
The operation of switch 2 shown in FIG. 2 is the following.
It is assumed that switch 24 is open. At the beginning of a positive halfwave, a current flows, from terminal 23, through a first diode of bridge 28, resistor R2, the gate and the cathode of thyristor 29, the diode of bridge 28 opposite to the first one, then from gate G to anode A1 of triac 21. As soon as the current reaches the value required to trigger thyristor 29, the latter turns on. Afterwards, as soon as the current flowing through thyristor 29 becomes sufficient to trigger triac 21, the latter triggers, thus short-circuiting all other components of switch 2. At the beginning of a negative halfwave, a current flows from terminal 22, from anode A1 to the gate of triac 21, through a third diode of bridge 28, from the cathode to the gate of thyristor 29, through resistor R2, then through a diode of bridge 28 opposite to the third one. As previously, switch 2 is triggered in two steps, by the turning-on of thyristor 29 short-circuiting resistor R2 and switch 24, then by the turning-on of triac 21.
A thyristor 29 is used in order to provide a component that is more sensitive to triggering than a triac. Indeed, it is known to implement thyristors (with a cathode-gate), having triggering current on the order of some hundred .mu.A, or even less. Accordingly, resistor R2 can be sized with a much higher value than in the switch shown in FIG. 1. As a result, when switch 24 is closed to prevent the triggering of thyristor 29 by short-circuiting its gate and its cathode, the power dissipated in switch 2 is much lower.
As a specific example, assuming that thyristor 29 has a triggering current of 100 .mu.A and taking, as a triggering voltage, the same value as previously (that is, 20 volts), resistor R2 can have a value on the order of 200 k.OMEGA.. As a result, the dissipated power when switch 24 is on is on the order of 100 mW.
If such a switch overcomes the dissipated power disadvantage of the switch of FIG. 1, it however has some disadvantages.
A first disadvantage is that it requires a high number of components due to the presence of diode bridge 28. Further, all these components have to withstand the high a.c. supply voltage (for example, approximately 220 volts).
Another disadvantage is that switch 24 is not referenced to a.c. supply voltage Vac but to the negative rectified voltage terminal of bridge 28. This makes the control of switch 24 more complex and limits the applications in which such a switch can be used. Indeed, a control circuit isolated from the mains then has to be used (a power supply with a transformer or using an optocoupler). This prevents the use of the same control circuit to control several switches.